Diffusion contrast is based on the self-diffusion of water molecules in tissue.
Although a variety of sequences are now used to acquire DW images,
all DW sequences include two equal and opposing motion-probing gradients.
Diffusion is anisotropic (directionally dependent) in WM fiber tracts,
as axonal membranes and myelin sheaths represent barriers to the motion of water molecules in directions not parallel to their own orientation.
The direction of maximum diffusivity has been shown to coincide with the WM fiber tract orientation .
This information is contained in the diffusion tensor,
a mathematic model of diffusion in three-dimensional space.
The tensor model of diffusion consists of a 3x3 matrix derived from diffusivity measurements in at least six noncollinear directions.
Fractional anisotropy (FA) is an index ranging from 0 (isotropic) to 1 (maximally anisotropic)) The direction of maximum diffusivity may be mapped by using red,
green,
and blue (RGB) color channels with color brightness modulated by FA,
resulting in a convenient summary map from which the degree of anisotropy and the local fiber direction can be determined.
The convention we used for directional RGB color mapping is red for left-right,
green for anteroposterior,and blue for superior-inferior.
We employed b value of b 0 and 1000 s/mm2 and 82 encoding gradient directions in the comparative study with anatomic sections from cadavers ,
meanwhile 25 encoding gradient directions directions were used when evaluating different clinical applications.
White matter tracts were estimated with tractography. Tracking was initiated from a start location (or seed point) in both forward and backward directions.
The ROIs were chosen to enclose tract cross sections that were visible in any of the axial,
sagittal,
or coronal directional color maps.
Fiber trajectories are displayed with colors superimposed on gray-scale anatomic images in various three dimensional projections.
Note that,
unlike directional color maps in which directional information is color-coded,
individual tractograms are displayed by using fixed colors which are arbitrarily chosen.(Figure 1),
(Figure 2),
(Figure 3)
WHITE MATTER FIBER CLASSIFICATION
• Association fibers interconnect cortical areas in each hemisphere.
Association fibers typically identified on DTI color maps include: cingulum,
superior and inferior occipitofrontal fasciculi,
uncinate fasciculus,
superior longitudinal (arcuate) fasciculus,
and inferior longitudinal (occipitotemporal) fasciculus.
• Projection fibers interconnect cortical areas with deep nuclei,
brain stem,
cerebellum,
and spinal cord.
There are both efferent (corticofugal) projection fibers (cortico-spinal,
cortico-bulbar,
and cortico-pontine) and afferent (corticopetal) projection fibers (medial lemniscus; anterior,
superior and posterior thalamic radiation; inferior spinocerebellar fascicles or Flechsig's fasciculus,
superior spinocerebellar fascicles or Gowers' tract and optic radiation).
• Commissural fibers interconnect similar cortical areas between opposite hemispheres (corpus callosum,
anterior commissure,
posterior commissure,
interthalamic gray commissure or intermediate mass and Psalterium or David ´s lyra).
ASSOCIATION FIBERS
Cingulum:The cingulum begins in the parolfactory area of the cortex below the rostrum of the corpus callosum,
then courses within the cingulate gyrus,
and,
arching around the entire corpus callosum,
extends forward into the parahippocampal gyrus and uncus.
It interconnects portions of the frontal,
parietal,
and temporal lobes.
Its arching course over the corpus callosum resembles the palm of an open hand with fingertips wrapping beneath the rostrum of the corpus callosum.
Superior Occipitofrontal Fasciculus: Whereas the cingulum wraps around the superior aspect of the corpus callosum,
the superior occipitofrontal fasciculus lies beneath it.
It connects occipital and frontal lobes,
extending posteriorly along the dorsal border of the caudate nucleus.
Portions of the superior occipitofrontal fasciculus parallel the superior longitudinal fasciculus but they are separated from the superior longitudinal fasciculus by the corona radiata and internal capsule (Figure 4).
Inferior Occipitofrontal Fasciculus: The inferior occipitofrontal fasciculus also connects the occipital and frontal lobes but is far inferior compared with the superior occipitofrontal fasciculus.
It extends along the inferolateral edge of the claustrum,
below the insula.
Posteriorly,
the inferior occipitofrontal fasciculus joins the inferior longitudinal fasciculus,
the descending portion of the superior longitudinal fasciculus,
and portions of the geniculocalcarine tract to form most of the sagittal stratum,
a large and complex bundle that connects the occipital lobe to the rest of the brain.
The middle portion of the inferior occipitofrontal fasciculus is bundled together with the middle portion of the uncinate fasciculus.
Uncinate Fasciculus Uncinate is from the Latin uncus meaning “hook.” The uncinate fasciculus hooks around the lateral fissure to connect the orbital and inferior frontal gyri of the frontal lobe to the anterior temporal lobe.
The anterior aspect of this relatively short tract parallels,
and lies just inferomedial to,
the inferior occipitofrontal fasciculus.
Its midportion actually adjoins the middle part of the inferior occipitofrontal fasciculus before heading inferolaterally into the anterior temporal lobe.
(Figure 5)
Superior Longitudinal (arcuate) Fasciculus:The superior longitudinal fasciculus is a massive bundle of association fibers that sweeps along the superior margin of the insula in a great arc,
gathering and shedding fibers along the way to connect frontal lobe cortex to parietal,
temporal,
and occipital lobe cortices.
The superior longitudinal fasciculus is the largest association bundle.
(Figure 6)
Inferior Longitudinal (occipitotemporal) Fasciculus: The inferior longitudinal fasciculus: connects temporal and occipital lobe cortices.
This tract traverses the length of the temporal lobe and joins with the inferior occipitofrontal fasciculus,
the inferior aspect of the superior longitudinal fasciculus,
and the optic radiations to form much of the sagittal stratum traversing the occipital lobe.
(Figure 7)
PROJECTION FIBERS:
Corticospinal,
Corticopontine,
and Corticobulbar: The corticospinal and corticobulbar tracts are major efferent projection fibers that connect motor cortex to the brain stem and spinal cord.
Corticospinal fibers converge into the corona radiata and continue through the posterior limb of the internal capsule to the cerebral peduncle on their way to the lateral funiculus.
Corticobulbar fibers converge into the corona radiata and continue through the genu of the internal capsule to the cerebral peduncle where they lie medial and dorsal to the corticospinal fibers.
Corticobulbar fibers predominantly terminate at the cranial motor nuclei.
(Figure 8)
The thalamic fibers (anterior,
superior,
and posterior thalamic radiations: The anterior thalamic radiation interconnects anterior and medial thalamic nuclei with the frontal lobe (Figure 9); the superior thalamic radiation joins ventral group of thalamic nuclei with the motor sensory areas and adjoining parts of frontal and parietal lobes (Figure 10).The posterior thalamic radiation joins occipital and posterior parietal cortex with posterior thalamus including pulvinar; it includes optic radiation from lateral geniculate body (Figure 11).
Geniculocalcarine Tract (optic radiation):The optic radiation connects the lateral geniculate nucleus to occipital (primary visual) cortex.
The more inferior fibers of the optic radiation sweep around the posterior horns of the lateral ventricles and terminate in the calcarine cortex; the more superior fibers take a straighter,
more direct path.
The optic radiation mingles with the inferior occipitofrontal
fasciculus,
inferior longitudinal fasciculus,
and inferior aspect of the superior longitudinal fasciculus to form much of the sagittal stratum in the occipital lobe (Figure 12).
Medial lemniscus: It is part of the posterior column-medial lemniscus system,
which transmits touch,
vibration sense,
as well as tthe pathway for propioception.
Axons of cells within nucleus gracilis and nucleu cuneatus cross as internal arcuate fibers and form the medial lemniscus.
The medial lemniscus carries axons from most of the body and synapses in the ventral posterolateral nucleus of the thalamus ,
at the level of the mamilary bodies.
Sensory axons transmitting information from the head and neck via the trigeminal nerve synapse at the ventral posteromedial nucleus of the thalamus.
The superior thalamic radiation connect the ventral posterolateral nucleus of the thalamus to the postcentral gyrus (somatosensory cortex) of the cerebral cortex (areas 3,
1,
2) (Figure 13).
Inferior spinocerebellar fascicles or Flechsig's fasciculus: The inferior spinocerebellar tract conveys inconscient propioceptive information from the body to the cerebellum.
Proprioceptive information is taken to the spinal cord via central processes of dorsal root ganglia (first order neurons).
These central processes travel throughth the dorsal horn where they synapse with second order neurons of Clarke's nucleus.
Axon fibers from Clarke's Nucleus convey this proprioceptive information in the spinal cord in the peripheral region of the posteriolateral funiculus ipsilaterally until it reaches the cerebellum,
where unconscious proprioceptive information is processed.
Superior spinocerebellar fascicles or Gowers' tract: The superior spinocerebellar tract conveys propioceptive information from the body to the cerebellum.
Originates from ventral horn at lumbosacral spinal levels.
Axons first cross midline in the spinal cord and run in the ventral border of the lateral funiculi.
These axons ascend up to the pons where they join the superior cerebellar peduncle to enter the cerebellum.
Middle cerebellar peduncles: The largest of the three paired peduncles,
composed mainly of fibers that originate from the pontine nuclei,
cross the midline in the basilar part of the pons,
and emerge on the opposite side as a massive bundle arching dorsally along the lateral side of the pontine tegmentum into the cerebellum.
Via this connection,
the cerebellum receives a copy of the information for muscle movement that the pyramidal tract is carrying down to lower motor neurons (Figure 14).
COMMISSURAL FIBERS:
Corpus Callosum: By far the largest WM fiber bundle,
the corpus callosum is a massive accumulation of fibers connecting corresponding areas of cortex between the hemispheres.
Fibers traversing the callosal body are transversely oriented,
whereas those traversing the genu and splenium arch anteriorly and posteriorly to reach the anterior and posterior poles of the hemispheres (Figure 15).
Anterior Commissure: The anterior commissure crosses through the lamina terminalis.
Its anterior fibers connect the olfactory bulbs and nuclei; its posterior fibers connect middle and inferior temporal gyri.
Posterior Commissure: The posterior commissure (also known as the epithalamic commissure) is a rounded band of white fibers crossing the middle line on the dorsal aspect of the upper end of the cerebral aqueduct.
It is important in the bilateral pupillary light reflex.
Interthalamic gray commissure or intermediate mass: The medial surface of the thalamus constitutes the upper part of the lateral wall of the third ventricle,
and is connected to the corresponding surface of the opposite thalamus by a flattened gray band,
the Interthalamic adhesion.
Psalterium or David ´s lyra: The lateral portions of the body of the fornix are joined by a thin triangular lamina,
named the psalterium (lyra).
This lamina contains some transverse fibers that connect the two hipoccampi across the middle line and constitute the commissure of fornix (hippocampal commissure) (Figure 16).
ROM A FUNCTIONAL STANDPOINT THE MOST IMPORTANT FASCICULUS ARE:
- Pyramidal tract (motor studies).
- Medial lemniscus and superior corona radiata (sensory studies).
- Meyer´s loop and optical radiations (visual studies).
- Arcuate and inferior frontooccipital fasciculilanguage assessment): Arcuate fasciculus lesions cause paraphasia phonetics (conduction aphasia) and inferior longitudinal fasciculus lesions cause semantic paraphasias.
CLINICAL APPLICATIONS :
There are many pathological processes that affect anisotropic diffusion.
The main clinical applications are:
Myelination process: In premature newborns,
increased anisotropy is found in developing cortical gray matter rather than in unmyelinated white matter,
and cortical anisotropy steadily decreases during the first few months of life.
DTI can assess the process of myelination.
Developmental brain disorders: DT I allows the study of abnormal interhemispheric connections in cases of complete or partial agenesis of the corpus callosum.
In lissencephaly,
tractography of the grossly abnormal subcortical and deep white matter has demonstrated an incomplete development of the fornix and cingulate tracts.
In cases of alobar holoprosencephaly ,
absence of corticospinal tracts have been observed by means of DTI .
Many white matter tract structures,
such as the middle cerebellar peduncles,
were found to be smaller in alobar holoprosencephaly than in semilobar or lobar holoprosencephaly.
Furthermore,
the size of the corticospinal tracts and middle cerebellar peduncles in all three variants was found to correlate with neurodevelopmental status.
Joubert syndrome shows aberrant connections in abnormal superior cerebellar peduncles (Figure 17).
Demyelinating disease: Diffusion tensor is an effective tool in the assessment of white matter involvement,
specially for the purposes of quanting the anisotropy and to perform to follow up studies in patients with multiple sclerosis.
DTI imaging allows to control the degree of cortico-spinal tract involvement (Figure 18) (Figure 19).
Hypoxic ischemic disease: Tractography is useful in assessing both the integrity of the different fibers in the ischemic area and prognosis: We use it to establish a prognosis in children with congenital hemiparesis valuing the FA in the cortico-spinal tract.
Thus a decrease <3% suggests mild hemiparesis,
decreased values ranging from 18%-46% suggest moderate hemiparesis ,
and values higher than 47% suggest severe hemiparesis.
In cases of cystic cavities of encephalomalacia or porencephalic.
wallerian degeneration occurs,
after stroke,
there is a decrease in FA values (Figure 20) (Figure 21) (Figure 22).
Infectious diseases: Encephalitis,
particularly herpetic encephalitis causes marked impairment of cortical and white matter with involvement of temporal lobes,
limbic system and insular region .
affecting inferior fronto-occipital fasciculus ,uncinate fasciculus,
inferior longitudinal fasciculus and cingulum (Figure 23).
Degenerative disease: Parkinson's disease presents reduced FA in the substantia nigra with normal anisotropy in caudate and putamen.
lncreased diffusivity and decreased anisotropy were found in the corpus callosum and the frontal,
temporal,
and parietal white matter in both patients with Alzheimer disease and those with Lewy body dementia,
but the occipital lobes were involved only in the latter.
Multiple groups have demonstrated decreased anisotropy and increased diffusivity in the internal capsule and cerebral peduncles of patients with amyotrophic lateral sclerosis .
In multisystemic atrophy (MSA) we can detect the degree of involvement of the middle cerebellar peduncles and brainstem (Figure 24).
Tumors: The goal of surgical treatment for cerebral neoplasms is to maximize the extent of tumor resection while minimizing postoperative neurologic deficits resulting from damage to intact,
functioning brain.
This requires preoperative or intraoperative mapping of the tumor and its relationship to functional structures,
including cerebral cortex and WM tracts.
Cortical mapping can be accomplished with either functional MR imaging or intraoperative electrocortical stimulation (Figure 25) (Figure 26) (Figure 27) (Figure 28) (Figure 29) (Figure 30) (Figure 31) .
Preoperative mapping of vascular disease: Surgery of cavernomas is the most common vascular indication (Figure 32) (Figure 33) .
Epilepsy: Tractography is particularly usefull in the assesment of surgical aproach,
specially in resections close to Broca and Wernicke areas,
as is capable of assessing the integrity of the arcuate and inferior frontooccipital fasciculus.
It also allows studying the integrity of optical radiation and Meyer loop in order to prevent visual field defects (figure 34).
Phychiatric disorders: Various disorders such as attention deficit disorder in obsessive compulsive disorders,
autism or schizophrenia show decreased FA in specific locations.